For various equipments, a gravity compensation force applied to an object may be beneficial if to move or rotate the object of mass. With a gravity compensation apparatus able to offset partially or completely the gravity, a smaller force can be applied to move or rotate the object. In addition, a simpler structure may be used in design because the mechanism to generate the required force is simpler, leading to a cost-down in the manufacturing expense.
FIG. 1 shows a schematic view of a conventional gravity compensation technique. Object 11 has a mass W, and is able to rotate around axis 12 as a center. The distance between the center of mass of object 11 and axis 12 is L. The gravity compensation mechanism is a torsion spring 13 installed at the center of axis 12. A fixed lever 131 of torsion spring 13 is fixed, and movable lever 132 rotate synchronously with object 11. Hence, torsion spring 13 is used to compensate or eliminate the effect of gravity during the rotation of object 11 from 0-90°. The spring torque coefficient of torsion spring is KM, and θ is the rotation angle of the object. Then,Torque of axis 12=W×L×sin θ,Compensation torque of torsion spring=kM×θwhere W, L are constant, and sine is a non-linearly increasing curve. Therefore, the obtained torque changing with θ is non-linearly increasing. However, torsion spring 13 is a linear spring providing a linear increasing compensation torque. Hence, this type of gravity compensation technique fails to satisfy the torque change caused by the gravity change.
To solve this problem, U.S. Pat. No. 6,899,308 disclosed a passive gravity compensating mechanisms, using two sets of rotational mechanisms with non-spherical surface, in combination with two sets of springs connected to the aforementioned mechanism to achieve gravity compensation. A preferred embodiment of the patent in structure is to use a pair of non-spherical gears, which leads to manufacturing difficulty and high cost. In addition, the restrictions of material strength (metal gear) and matching precision are more prominent when miniaturizing the product.
U.S. Pat. No. 7,677,523 disclosed a gravity compensation device. The disadvantage of the structure of the disclosed gravity compensation device is that the structure fails to simulate the gradually decreasing part of the trigonometry slope. If the radius of gear retracts and the resilient cable fails to contact the retracted part, the cable will be pushed by the relatively protruding front part. Also, the distance between the load and the gravity compensation device changes as the radius of the gear changes, or the angle to apply the force changes when the distance remains the same.